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CAN Bus with automatical bus termination

Discussion in 'Embedded' started by Markus Forrer, Oct 6, 2004.

  1. For a new device generation communicating with CANopen I d'like to implement
    automatical bus termination, because the actual dip switch for manual bus
    terminantion is a big problem for our support!

    Idea: Each module has a CAN input connector and a CAN output connector. A
    electronic switch toggle between bus termination of CAN input (boot-up
    position) and connect through CAN input to CAN output.

    After boot-up, the master identify the first module and order the module to
    toggle the switch. The module switch the CAN bus through to the next module
    (if exist) and the bus termination is switched off. If an other module can
    be find by the master, the switch has to stay in the actual position.
    Otherwise the switch change after a timeout time to the default position
    (bus termination) and the hole CAN bus is properly terminated.

    What about this idea? Is there a standard to implement this procedure?

    Thanks very much!

    Markus
     
    Markus Forrer, Oct 6, 2004
    #1
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  2. Hmmm... why do it on the module at all? Wouldn't it be a heck of a
    lot simpler to do termination as a dummy plug on the output connector,
    given you're already willing to spend plugs where a single one should
    do? Or, if you really have to, use an output jack with a switch in it
    that is operated by inserting a plug, like those headphone jacks that
    automatically switch off the main speakers usually found on boom-boxes
    and notebooks.

    Conceptually, termination is a feature of the *bus*, not of the
    individual node, so IMHO it should be handled by mechanical design of
    the cabling. A terminator on the node, whether controlled by a DIP
    switch or a CAN command, can only ever be a second-best method.

    And that's before you start considering that CAN is a bus without a
    physically preferred direction of travel, so your node can't really
    know which of its two plugs is the "input" and which the "output". Or
    that you would have to actually *disconnect* the cable on the "output"
    side for your trick with electronically controlled termination inside
    the node to work --- otherwise the (still active) terminators of the
    other nodes further down the bus would cause no end of troubles. Now,
    what if the node is plugged in "backwards", i.e. the cable leading to
    the master, which you called "input", happens to be plugged into the
    "output" socket? A guaranteed non-working system.
     
    Hans-Bernhard Broeker, Oct 6, 2004
    #2
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  3. Thanks for your answer!

    Some more informations:

    1. The input and output connectors have to be different types to avoid
    illegal connecting the plugs.

    2. If the switch is in boot-up position, the bus termination is enabled
    _and_ the CAN output connector ist disconnected (toggle switch). So the
    termination of the next module will not give some troubles.

    An other motivation for this idea is the information of module position
    inside the CAN bus I'll be able to detect. In my application I should find
    out which module is next of CANopen master (#1), which is after that (#2)
    and which is the last module (#n). Otherwise each module has to be configure
    manually (present solution non user-friendly).

    Thats also a good idea! But I need an industrial connector and I've never
    seen any with switch in it...

    I now, a really disadvantage is the non standard of my idea! :-( But I'm
    searching for a user-friendly solution for the themes 'bus termination' and
    'order of modules'!

    Thanks very much!

    Markus
     
    Markus Forrer, Oct 6, 2004
    #3
  4. Not necessarily.

    IIRC, CANopen is supposed to be using software protocols to handle
    node address assignment instead. Assigning node numbers by cable
    positions seems like the wrong thing to do. It links two design
    decisions (cable routing and CAN node numbering) together which should
    really be independent of each other, thus reducing your flexibility.

    If you need a method to select a given CAN node by its function, give
    it a physical ID (e.g. a Maxim 1-wire "serial number" thingy that can
    be read out via CAN-bus).
    That still doesn't rule out the "terminator plug" idea, though. I.e.
    make a rule that all jacks must have something plugged into them, and
    provide two kinds of things to be plugged in: a cable with two
    different plugs, and an isolated terminator plug that fits into output
    jacks only.
     
    Hans-Bernhard Broeker, Oct 6, 2004
    #4
  5. Not necessarily.

    In my application it is necessary! Not for the CANopen communication, but
    for the business himself! There are severeal identical I/O modules and the
    CANopen master must recognize, which input ON WHICH PHYSICAL POSITION on the
    bus has toggled! In the present the service engineer has to configure each
    module: module next of CANopen master setting to #1 and so on. After
    replacing a module in fact of defect the same activity!

    Now I'd like to simplify that! I'd like to have a table in my software
    application like that: serial number of module (for LSS) <-> node ID <->
    physical position on the bus.

    Rightly! But in the meantime I now the service engineers: '...why I have to
    plug in this silly plug without cables? It works well with out!" (the
    question is: how long) ;-)
    Or: "...uuups, I loose this funny plug! What the hell!"...

    Thanks for your tips!

    Markus
     
    Markus Forrer, Oct 6, 2004
    #5
  6. But with your cable selection scheme, you get the same kind of
    problem, if not a worse one, as soon as you have to add or remove any
    device: unless you're lucky and this happens at the end of the bus,
    you'll modify half or all the numbers of the existing devices, when in
    fact you changed only one. That's why a board identifier is better
    than cable-addressing: it changes only if and when you exchange the
    actual CAN module doing a given job (which should not be necessary
    unless that particular module broke), and it leaves all the others
    alone.
    Well, if they want to be stupid, there's nothing you can do about it
    anyway. Let them. Just make sure you have a nice hardcopy printout
    of the manual to whack over their head if they come to you with this
    kind of self-induced problem.
     
    Hans-Bernhard Broeker, Oct 6, 2004
    #6
  7. Markus Forrer

    H.-J.Oertel Guest

    Without following the whole thread, this is not CANopen anymore.
    Using two differnet types of connectors is ugly.
    DS 303-something (1) defines different types, but assumes if you have
    different types, they should be the same for in/out, right/left or whatever
    it is called. Look at the typical CANopen nodes using the very same DSUB-9
    kind of thing.
    --

    with best regards / mit freundlichen Grüßen

    Heinz-Jürgen Oertel
    +===================================================================
    | Heinz-Jürgen Oertel port GmbH http://www.port.de
    | mailto:
    | phone +49 345 77755-0 fax +49 345 77755-20
    | Regensburger Str. 7b, D-06132 Halle/Saale, Germany
    | CAN Wiki http://www.CAN-Wiki.info
    | Newsletter: http://www.port.de/engl/company/content/abo_form.html
    +===================================================================
     
    H.-J.Oertel, Oct 6, 2004
    #7
  8. Just make sure you have a nice hardcopy printout
    :))

    Thanks!

    Markus
     
    Markus Forrer, Oct 7, 2004
    #8
  9. Without following the whole thread, this is not CANopen anymore.
    You'r right! First I'll study the CANopen specifications...

    Thanks for the answer!

    Markus
     
    Markus Forrer, Oct 7, 2004
    #9
  10. I don't know of any such standard.
    But
    <shameless plug>
    you can buy such modules at my company, see e.g.
    http://www.iep.de/Hauptebene/Hardware/CANIO/canio.htm
    (only in german right now, sorry). The english prospectus at
    http://www.iep.de/Hauptebene/Prospekte/English/CanIO-Module.pdf#
    doesn't mention this feature, as it's rarely requested.
    </shameless plug>

    We implemented this because
    a) we (and some of our customers) believe that it eases replacement
    with spares
    b) sometimes you can identify and locate modules with defect
    interfaces or cable problems.

    Andreas
     
    Andreas Hadler, Oct 7, 2004
    #10
  11. Hi Markus,

    I can confirm from several of our clients that for their way of using
    CANopen this would be a very desirable functionality. Unfortunately,
    the switch has another problem. Every switch adds a little noise
    and/or delay to the signal. And with CAN it is quite crucial to
    specifically avoid any delays...

    So what you are suggesting might reduce the total number of nodes that
    you can connect to a segment or reduce the maximum length of the
    cabling that you can use.

    In case you are still interested in a solution for auto-termination
    only: that is quite easy to do with Ethernet sockets that can detetct
    if a cable is plugged in or not. You just have to wire the termination
    resistor in a way that it is automatically "hot" if only one cable is
    inserted, but it gets removed when you plug-in both.

    Olaf
    Tutor at ESAcademy dot com
    www.CANopenBook.com
     
    Tutors of ESAcademy, Oct 19, 2004
    #11
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